For example, JP-A No. 42220/1989 discloses a conventional method of lining channels. With this conventional method, a rigid or semirigid plastic pipe shaped initially in the form of a round tube is flattened, then folded in two and wound up in a folded U shape on a roll when produced in a plant. At a work site, the pipe is inserted into a channel while being paid off from the roll in a state softened by heating. The inserted pipe is inflated by applying heat and pressure thereto from inside for restoration to the round tubular form as shaped initially, forming a lining along the inner surface of the channel with the rigid or semirigid plastic pipe.
According to the lining method described, the pipe is diminished in effective outside diameter by being deformed to the folded U shape and can therefore be inserted into the channel if not fully satisfactorily, whereas since the pipe memorizes the initial shape, the U-shaped folded portion tends to become unfolded as indicated in broken lines in FIG. 6 to correspondingly increase the effective outside diameter when the pipe is so softened by heating as to be paid off from the roll, with the result that the pipe encounters increased resistance when inserted into the channel, hence a reduced work efficiency. Especially if made of polyethylene, the pipe remains somewhat elastic even when softened by heating, consequently exhibiting a marked tendency to restore its shape before deformation when released from the restraint of the deforming step.
Further if the pipe is held wound on a roll in the folded U shape before use, a folding tendency is afforded in the U-shaped folded portion of the pipe during the preservation period. The folding tendency partly becomes a permanent deformation when the pipe is inflated for restoration, preventing restoration to the form as shaped initially.
Since the bottom part X inside the U-shaped portion of the pipe (see FIG. 6) is rounded in a direction opposite to the restoration direction Y, the rounded part needs to be reversed by inflation for restoration, so that the inflation entails a considerable adverse effect. If the bottom part X has such a folding tendency and when the rounded part is reversed by inflation for restoration, numerous minute cracks are likely develop to impair the quality of the lining undesirably.